1. Field of the Invention
The present invention relates to apparatus and methods for use in the flotation separation of particles from a fluid particulate suspension. More particularly, the present invention relates to air-sparged hydrocyclone flotation separators wherein hydrophobic particles in the fluid suspension are removed therefrom in a foam.
2. The Prior Art
(a) Flotation Systems
Flotation is a process in which one or more specific particulate constituents of a slurry or suspension of finely dispersed particles become attached to gas bubbles to enable separation of those constituents from the others of the slurry or suspension. The buoyancy of the particle/bubble aggregate formed by the adhesion of the gas bubble to a particle in the slurry is such that the aggregate rises to the surface of the separation vessel, where it may be then separated from the remaining particulate constituents which are yet in the aqueous phase of the suspension.
Flotation techniques can be applied where conventional gravity separation techniques are difficult to apply or not economical. Indeed, flotation has supplanted gravity separation methods in solving a number of separation problems. Originally, flotation was used to separate sulphide ores of copper, lead, and zinc from associated gangue mineral particles. Flotation is now also used for concentrating nonsulphide ores, for cleaning coal, for separating salts from their mother liquors, and for recovering elements, such as sulphur and graphite.
During the past two decades, the application of flotation technology to mineral recovery in the United States has increased at an annual rate of about 7.4%. Present flotation installations in the United States alone are capable of processing almost two million (2,000,000) tons of material per day.
The preferred method for removing the floated constituent of a fluid suspension of particles is to form a froth or foam of the collected particle/bubble aggregates. The froth can then be removed from the top of the suspension. This process, which may be conducted as a continuous process is called froth flotation. The effectiveness of froth flotation is enhanced by the introduction into the separation vessel of voluminous quantities of small bubbles, typically in the range of about 0.1 to about 2 millimeters in diameter.
In conventional processes, the success of flotation has depended upon controlling conditions in the particulate suspension so that small air bubbles are selectively attached to one or more particle constituents, while not being attached to the other particle constituents of the suspension. To achieve this selectivity, the slurry or particulate suspension is typically treated by the addition of small amounts of known chemicals or flotation enhancing regents which selectively render one or more of the constituents in the particulate suspension hydrophobic. Chemicals which render hydrophobic a particulate constituent which is normally less hydrophobic or even hydrophilic, are commonly referred to as "collectors," while those that increase the hydrophobicity of a somewhat hydrophobic particulate constituent are referred to as "promoters."
Treatment with a collector or promoter causes those constituents rendered hydrophobic to be repelled by the aqueous environment and attracted to the air bubbles therein. The hydrophobic nature of the surface of these constituents enhances the attachment of air bubbles to the hydrophobic constituents. Thus, control of the surface chemistry of particulate constituents by the addition of flotation enhancing reagents, such as collectors or promoters, allows for the selective formation of particle/bubble aggregates with respect to those constituents.
Other chemicals or flotation enhancing reagents may be used to help create the froth phase for the flotation process. Such chemicals are commonly referred to as "frothers." The most common frothers are short chain alcohols, such as methyl isobutyl carbinol, pine oil, and cresylic acid. Important criteria related to the choice of an appropriate frother include the desired solubility and collecting properties of the froth, such as its toughness, texture, and breakage characteristics. The size, number, and stability of the bubbles during flotation may be optimized at a certain frother concentration. An appropriate frother thus ensures that the froth will be sufficiently stable to sustain the particle/bubble aggregates through removal as a flotation product. Frequently the mixture of desired mineral product and other entrained minerals which are present in the froth is referred to as a concentrate. A proper froth should allow for the drainage of water and for the removal of misplaced hydrophilic particles from the froth.
A complete flotation process is thus conducted in several steps. First, a slurry is prepared containing from about five percent (5%) to about forty percent (40%) by weight of solids in a fluid, usually water. Second, the necessary flotation enhancing reagents are added and agitated with the slurry to distribute the reagents on the surface of the particles targeted to be removed by flotation. Third, the treated slurry is aerated in a separation vessel by agitation in the presence of a stream of air or by distributing the air in fine streams as bubbles through the slurry to produce a froth of particle/bubble aggregates involving the target particles. Finally, the target particles are withdrawn from the top of the cell as concentrate or flotation product. The remaining solids and water are discharged from the bottom of the separation vessel.
One of the problems with conventional flotation methods is the lengthy slurry retention time required in the separation vessel of at least two minutes to achieve successful separation. Relatively long retention times limit plant capacity and necessitate the construction of extremely large equipment at the expense of floor space and capital.
(b) Cyclonic Separators
Cyclonic separators utilize fluid pressure energy to create rotational fluid motion. This rotational motion causes relative movement of the particles suspended in the fluid, thereby permitting separation of particles, one from another or from the fluid in the manner of a centrifuge. These devices are occasionally referred to merely as hydrocyclones.
The rotational fluid motion is produced by the injection of fluid under pressure into a separation vessel. At the point of entry for the fluid, the vessel usually has walls that are cylindrical. The walls may remain cylindrical over the entire length of the vessel, though it is more common for a portion of the vessel to be conically shaped. Nevertheless, as used herein the term "generally cylindrical" as applied to the walls of a hydrocyclone is intended to include such side walls as are wholly or partially cylindrical.
Hydrocyclones may be used successfully for dewatering a coarse suspension or for making a size separation between the particulates in the suspension. In this case the device is called a classifying hydrocyclone. Equally important, however, is the potential for the use of hydrocyclones for gravity separation. Hydrocyclones have been used extensively as gravity separators in coal preparation plants. Design features have been established for such applications which emphasize the difference in the specific gravity of particles rather than differences in particle size.
One of the types of hydrocyclones used for gravity separation has four inlet/outlet ports and consists of a straight-wall cylindrical vessel that may be operated at various inclination ranging from horizontal to vertical. A fluid particulate suspension, or slurry, enters the vessel through a coaxial feed pipe, generally at the upper end of the vessel. A second fluid, typically water or a heavy media suspension, is injected under pressure tangentially into the vessel through an inlet adjacent the lower end of the vessel. The second fluid mixes with the fluid particulate suspension and creates a completely open vortex within the vessel as it transverses the length of the vessel toward a tangential reject discharge adjacent the upper end. The cyclonic action in the vessel separates the heavier particles from the fluid mixture for removal from the vessel through a coaxial outlet or vortex finder at the lower end of the vessel.
(c) Air Sparged Hydrocyclone Separator
The principals of air-induced flotation separation may be employed in the environment of the hydrocyclone. The result is the air-sparged hydrocyclone.
In air-sparged hydrocyclone flotation, a separator vessel is employed having generally cylindrical walls. Portions or all of those walls are porous and surrounded on the outside thereof by an air plenum. Through this structure, pressurized air broken into small bubbles can be introduced into the separator vessel through the walls thereof. Alternately, but toward the same end, air in the form of small bubbles can be introduced into the particle suspension in other manners, such as by injecting air into the feed stream of the particle suspension before it reaches the separation vessel.
The slurry is fed into the separator vessel tangentially to the walls thereof through a conventional cyclone header. A forced vortex swirl flow develops on the inside surface of porous walls of the separator vessel. Pressurized air passes through the jacketed porous walls entering the separator vessel and is sheared into numerous small bubbles by the swirl flow of the slurry. Hydrophobic particles in the slurry collide with these bubbles, attaching to form particle/bubble aggregates. After attachment, the particle/bubble aggregates, have a relatively low specific gravity. The aggregates lose their tangential momentum and migrate radially inwardly to the center of the separator vessel, there forming a froth. This process is described in additional detail in U.S. Pat. Nos. 4,279,743, 4,397,741, 4,399,027, 4,744,890 and 4,834,434, which are incorporated herein by reference.
The froth is stabilized and constrained from sagging into the outlet area of the separator vessel by a froth pedestal. The froth continues to be generated, moving axially in the separator vessel towards a vortex finder at the end of the separator vessel opposite the froth pedestal. The froth is discharged through the vortex finder as an overflow product. Most hydrophilic particles remain in the slurry and are discharged together therewith as an underflow product through the annulus created between the sides of the froth pedestal and the wall of the separator vessel.
With such a design, effective flotation is possible that requires only a short retention time in the air-sparged hydrocyclone. For a typical design, the specific capacity of the air-sparged hydrocyclone separator is 100 to 600 tpd to per cubic foot of cell volume. As a result of such a high processing capacity, the retention time in an air-sparged hydrocyclone separator is very short, less than one second for the nominal two-inch diameter system. Despite such a high specific capacity, however, the flotation separation efficiency, that is the purity of the flotation froth product, may be difficult to sustain in certain cases.
Although most hydrophilic particles are rejected through the underflow annulus, some of these particles, which are usually gangue particles, inevitably migrate into the froth and are thus discharged together with hydrophobic particles through the vortex finder. This disadvantageously lowers the grade or purity of the froth product. In a short retention time, it is difficult to remove all these undesirable, entrapped hydrophilic particles from the froth product.